The present invention relates to the making of a mold for continuous casting using particularly contoured wear-proof lamina, inserts or the like and cooperating in conjunction with mold walls or wall plates made of copper or copper-containing material, said walls or plates being held within a frame and being tensioned in relation to each other for establishing the mold cavity whereby, particularly, the wear-proof elements are applied in some fashion to the mold walls and are fitted into them.
Continuous casting of metal with a high melting point, such as iron or steel, uses, for example, particular types of molds made of copper or copper-containing material becuase copper was a very high thermal conductivity, which high conductivity is needed for the rapid removal of heat from the molten metal so as to permit the formation of a solidifying skin. Depending upon the particular field of use, single piece molds have to be distinguished from multiple part molds. In the case of a single piece mold, one will use seamless, forged blocks or seamlessly pressed or cast tubes or one will use welded together sheets, skelp or strip. In the case of a multiple part mold, one will use certain wall plates arranged around a mold cavity and they are tensioned to each other within a frame. These plates have to be particularly thermally treated and are subject to certain deformations.
Molds for continuous casting of the type referred to above and made basically of copper, including low or high alloyed copper alloys, will in all instances undergo friction as far as the interior wall surface defining the mold cavity is concerned. The friction is particularly exerted upon the mold by the solidifying and solidified skin, including slag particles which lodge in between the casting strand and the mold walls. This wear on account of friction amounts, in effect, to a gradual change in the geometry of the mold, particularly the internal dimensions thereof, which in turn reduces the use life of the mold to a noticeable degree. Thus, it is necessary after a certain operating period to refinish these plates in order to counteract the mechanical and/or thermal wear which the mold walls have undergone to the detriment of the relevant geometry.
The refinishing operation modifies the original cross-section of the cavity so that it is inevitable that the casting strand has slightly different dimensions before and after the refinishing work. Thus result, however, is hardly tolerable because the cross-section of the casting will vary accordingly. On the other hand, the continuous casting process is a critical one and here, particularly the rate of skin formation, the internal pressure of the liquidous metal, the withdrawl speed, and the pouring speed into the mold are all critically interrelated and even a slight deviation from the expected norm may immediately lead to a rupture of the very thin skin, just a little downstream from the bottom of the mold. Any skin rupture is, within that particular field, always rather catastrophic. Another factor to be considered is that the machine, as such, establishes a particular path for the strand from the point of emergence from the mold towards the usual horizontal transport path along which the solidified casting is removed. While other operating parameters can readily be varied, that path is a pre-established one.
A specific problem in the casting of slab ingots, billets or the like are ruptures along the edges or corners. In order to offset this defect it has been proposed to round the corners of the cavity. In particular, then, in case of molds made of plates for continuously casting ingots of relatively large cross-section, one has to round the plates or provide them with rounded edges accordingly. Considering the wear above, one can readily see that not only the flat parts of such a wall surface have to be refinished but the rounded, corner-defining portions have to be refinished also, which is a further complication.
In view of the fact that a refinished mold will without further features exhibit a larger cross-section after refinishing and reassembly than before, one needs some form of adjustment. Readjusting the mold sizes and dimensions is a very complicated procedure and is particularly time consuming. Consequently an extensive down time of the machine will have to be tolerated or a larger inventory of spare mold parts is needed.
In order to overcome these difficulties and drawbacks outlines above, it has been proposed in German printed patent application No. 1939777 (see also U.S. Pat. No. 3,662,814) to arrange particular transition pieces in the corners between the wall plates; i.e., between a longitudinal plate and a transverse plate of the mold, transition pieces are interposed, being made of a material that in some form is different as compared with the material of which the mold plates are made. These transition pieces are mechanically fastened in recesses of the two intersecting wall plates whereby the abutment surface of such a transition piece in longitudinal direction of these plates is smaller than the thickness of the respective adjoining second and intersecting wall plate. However, it was found that the high throughput and the requirements for capacity in installed machines for continuous casting are too strong to be met by these kinds of molds. This is particularly true for molds having an invariable, i.e., from the outside unadjustable cross-section of the respective cavity. But even in cases where a mold's walls are adjustable, for example, in the type of molds wherein the cross-section is varied during the casting, for example, by way of shifting the small sides, one encounters friction on the engaging surfaces between the various mold walls and they experience a very strong wear. For example, galling was observed in that grooves were actually carved into the wall. This means that such a mold wall assembly will exhibit irregular gaps and will therefore become unusable in a very short period of time, which in turn means that refinishing work has to be caried out soon after the mold has been put into use.
In order to avoid strong friction and to therefore reduce the wear, it has been tried to coat the inside wall of the copper mold plates with higher resisting material such as nickel, chromium, molybdenum, or the like. Such a coating was provided only on the longitudinal or long side walls of the mold, while the transverse or short side plates remained uncoated. Here, thin, it was suggested to provide additional lubricating grooves along the edges of engagements and to fill these grooves with a highly heat resisting grease. This method of counteracting strong friction is disadvantaged by high cost, particularly on account of the additional coating. An additional drawback is to be seen in that the mechanical wear of the casting strand does affect, to some extent, this coating, which will abrade, even though at a lesser degree, but it still experiences wear. This wear is particularly noticeable because the wear-resisting coating should be quite thin. Any such coating has to be thin simply because it constitutes a heat transfer impediment, thereby in effect raising the temperature of the casting strand and retarding the formation of a skin. Other attempts have been made to coat the transverse or small side mold plates with a hard material, for example, by means of electroplating or a chemical deposit of nickel or by applying hard metals through flame or plasma spraying. However, the results have been minimal. A problem that is encountered on employment of such coatings, particularly on the small side coatings, is, for example, an insufficient adhesion. This means, for example, that during adjusting of the mold these coatings simply chip off or spall from the wall. Moreover, such spalling is undefined and may occur very quickly after the mold has been installed. Aside from the ensuing costs for repair and renewing the coating or cover, the undefined formation of gaps between assembled wall plates are a phenomenon that occur unforeseeably and suddenly, resulting in perforations and rupture of the casting strand which, of course, is dangerous and costly.
Independent from the foregoing, and particularly independent from the type of mold, be it one of fixed cross-sectional dimension or with adjustable ones, another area where high wear is observed is the immediate exit end of the mold. There is always strong friction between the solidified skin surface of the strand and the lower end of the mold walls. In fact, this strong friction limits severely the use life of the mold. In accordance with a known proposal made in German Patent No. 31 42 196, a wear protection in this particularly endangered area can be provided by protecting this area through a layer of greater wall thickness and to apply this layer by electroplating, by spraying, or by explosion cladding. But for similar reasons outlined above this approach has not proven to be useful.
Other types of wear and resulting problems are observed along the mold wall directly underneath the surface level. On this account, the German printed patent application No. 19 57 332 proposed to place inserts into the mold wall in the range of the surface level of the bath, the insert being made of a material different from the mold wall defining plates. It was found, however, that the mere placement of such inserts--for example, through hot rolling, cladding, high speed deformation or explosion cladding--is very expensive.